It is known to react aromatic diisocyanates with sulphur trioxide, oleum or sulphuric acid to produce monosulphonic acids of the isocyanates. U.S. Pat. No. 3,826,769, for example, describes a process for the sulphonation of tolylene diisocyanate in which the diisocyanate is mixed with less than a molar equivalent of sulphur trioxide. In this process, the sulphonation product precipitates from the liquid starting isocyanate in crystalline form.
The partial sulphonation of liquid mixtures of diisocyanates and polyisocyanates is described in German Offenlegungsschriften Nos. 2,227,111; 2,359,614 and 2,359,615 (U.S. application Ser. Nos. 363,436 and now abandoned; 527,473; and U.S. Pat. No. 3,959,329). In these cases precipitation of solid sulphonation product does not generally occur. Rather liquid polyisocyanates which contain sulphonic acid groups and which have valuable commercial properties are formed. The isocyanates which are to be sulphonated are mixed with from about 0.1 to about 32% by weight or, in extreme cases, with up to about 40% by weight of sulphur trioxide or an equivalent quantity of oleum, sulphuric acid or chlorosulphonic acid. This means that invariably less than 1 mol of sulphur trioxide is used for 1 mol of isocyanate. These sulphonation products will generally contain a maximum of 11.4% by weight of sulfur.
According to an earlier proposal (German patent application No. P 25 24 476.2), aromatic uretdione-diisocyante disulphonic acids are prepared by the reaction of aromatic dinuclear diisocyanates with sulphur trioxide. The molar ratio of dissocyanate to sulphur trioxide given for this reaction is between about 1:1 and 1:1.4, but even if a ratio above or below this range is employed the reaction products consist mainly of uretdione-diisocyanate disulphonic acids, in other words products containing one sulphonate group per monomer unit. The sulphur content of the products is about 8 to 10%.
Isocyanates having two or more sulphonic acid groups per monomer unit have not hitherto been known. Their formation has not been observed under the conditions hitherto employed for sulphonation.